The present invention relates to a transmitter for high frequency signals, which has two parallel signal branches with respective phase modulators and amplifiers, in which a dividing network that divides a received signal into both signal branches and a combining device that combines the output signals from both signal branches into a single sum signal are provided.
The linearity requirements of transmitter amplifiers have become increasingly greater due to the growing use of high stage digital modulation methods (e.g. 64-QAM or 256-QAM) in communication. Transmitter amplifiers in current radios are operated well under their saturation power in order to avoid degradation of the transmitted signal by non-linear distortions. The required output-back-off (i.e. the ratio of average transmitted power to saturation power) amounts to more than 9 dB in 64-QAM systems. Since the peak values of the transmitted signals in these systems are up to about 9 dB over the average value, a strict linear transmission behavior for the transmission amplifier is required over this modulation region. In order to linearize the transmission behavior of the transmitter amplifiers, a pre-distorter can be used. Since however a complete linearization up to the saturation power is not attainable with reasonable effort in practice, a further back off of a few dB must be provided. Non-ideal transmission efficiency thus results because of the undesirable ratio of average transmitted power to saturation power. This leads to high transmitter power losses and also to correspondingly high component and thus manufacturing costs.
A process, in which the transmitter linearity requirements can be avoided up to saturation, is known in the literature under the name LINC (Linear Amplification with Nonlinear Components). A linearized transmitter operating according to the LINC principle is known from D. C. Cooks, "Linear Amplification with Nonlinear Components", IEEE-COM, December 1974, pp. 1942 to 1945.
As described therein, a transmitter operating according to the LINC principle has two parallel signal branches with respective phase modulators and amplifiers. An input side dividing network produces two control voltages from the received signal for both phase modulators so that a sum signal with the desired amplitude and phase is produced in an output side combining device. The vector diagram shown in FIG. 1 shows, for example, the form of two sum signals SS and SS'. The sum signal SS or SS' arises by addition of the output signals S11 and S22 or S11' and S22' of both signal branches. Both output signals S11 and S22 or S11 ' and S22' are phase modulated and contain no amplitude modulation. Because of this they can be brought to the desired signal level in both signal branches with two amplifiers operated in saturation, without the intermodulation products occurring. The output signals S11 and S22 or S11' and S22' are brought into an arbitrary phase relationship so that a sum signal SS or SS' is provided with any arbitrary phase and amplitude.
As the vector diagram in FIG. 1 shows the output signals S11 and S22 or S11' and S22' have equal amplitudes. A rotation of the signal state in a central region of the signal state diagram occurs, e.g. during an amplitude error, also during an opposite side variation of the amplitudes of both output signals. In contrast rotation of the entire signal-state-diagram grid is observed for large output signal amplitudes. A rotation or compression of the grid in the central region of the signal state diagram and a rotation of the signal state at the corner point occurs, above all, in a phase error, also a deviation of the phases of both output signals. If a deviation of the actual signal state from the set or desired signal state in the signal state diagram of less than 1% is desired, the amplitude error may not exceed 0.1 dB and the phase error may not exceed 1%. These prerequisites are not achieved with the known LINC transmitters, chiefly because the phase modulators in both signal branches have temperature and age-dependent phase changes. However all other components of the transmitter also have temperature and age-dependent changes which lead to amplitude and phase errors.